A natural gas hydrate (NGH) has the advantages of large reserves, wide distribution, high energy density, cleanness, environmental friendliness and the like, and is considered as the most important clean and alternative energy in the 21st century, thus an NGH research has important scientific and practical significance.
The NGH research includes such aspects as resource investigation and evaluation, exploitation technology, safety and environmental impact and the like. On the basis of resource investigation and research, economic, efficient and safe NGH exploitation technology is a decisive factor in NGH resource development. Contents involved in the research on the NGH exploitation technology mainly include drilling, decomposition, gas production, environmental impact and the like. The NGH drilling technology is the basis and premise of achieving NGH exploitation. At present, reports on simulation research of NGH drilling are few. Although certain permafrost region and marine NGH field exploration sampling drilling and a small amount of test exploration drilling work have been carried out, NGH exploration sampling drilling and production exploration drilling are greatly different, therefore, a research on the NGH exploration drilling technology is crucial to the development and utilization of the NGH resource.
Since the natural gas hydrate is a matter strongly constrained by the environment, the formation and stability thereof require very special high pressure and low temperature environments, in an NGH drilling process, a large amount of heat is generated in a rock cutting process of a drill bit and by the friction of a bottom hole drilling tool with a well wall and a rock core, and formation stress near the well wall and the bottom hole is released, which will decompose NGH to produce a gas and decomposed water. The NGH decomposition causes serve damages to the drilling quality, the drilling speed, the equipment and the like. On one hand, after entering drilling fluid, the gas circulates with the drilling fluid to reduce the density of the drilling fluid, resulting in reduced hydrostatic pressure of the bottom hole and acceleration of the NGH decomposition, and a vicious circle is formed to eventually lead to the decomposition of a large quantity of hydrates in the bottom hole, resulting in such accidents as severe borehole diameter expansion, blowout, borehole collapse, casing deformation, ground settlement and on the like. On the other hand, when drilling in deep sea and frozen earth areas with very low temperatures, temperature and pressure conditions for re-forming the NGH by gas exist at a certain position in a well bore or in a ground pipeline, in this case, the NGH is likely to form in the drilling fluid to block the drilling fluid circulation (similar to natural gas hydrate blockage in an oil and gas transmission pipeline) or other pipelines of a drilling system, resulting in a series of serious accidents in the well. Therefore, it is a key issue related to the development and utilization of the NGH resource that whether bottom hole heat (temperature), pressure and NGH decomposition in the drilling process can be controlled. In addition, since submarine NGH exists in shallow sediments, a hydrate reservoir is weak in geological mechanical property and is low in rupture pressure, so that if the drilling pressure is too high, formation breakdown is generated to result in leakage of the drilling fluid. Therefore, compared with conventional oil and gas exploration drilling, the NGH drilling is very different on such aspects as drilling speed, drilling fluid ratio, pressure change, circulation velocity, bottom hole pressure control method, etc. Before mature and systematic NGH formation drilling theory and related technology are researched and established, if the conventional oil and gas exploration drilling technology is rashly adopted for drilling, unpredictable and uncontrollable safety accidents may be induced.
NGH exploration drilling methods can be divided into three categories: laboratory simulation, numerical simulation and field test, wherein the field test is costly and is only suitable for countries having found NGH physical samples; although the numerical simulation is low in cost, but must be based on basic data and basic laws obtained by the laboratory simulation; while the experimental simulation is to establish experimental simulation instruments and equipment in a laboratory and controls such conditions of the equipment as temperature, pressure, medium and the like to approximately simulate a natural NGH reservoir environment and research the generation, the drilling process rules and the influence mechanism. Since the experimental simulation is low in research cost and is the foundation of other researches, the NGH drilling experimental simulation research becomes the most feasible research method in the current NGH drilling technology research.
At present, the bottleneck problem restricting the development of the NGH drilling experimental simulation research is lack of a detection method and an experimental apparatus for quickly and accurately measuring the phase change and existing characteristics of the NGH in the drilling process in real time and in situ under high pressure and low temperature, this is mainly because the NGH formation conditions (high pressure and low temperature) are harsh and the experimental media are complicated, such that the existing oil and gas drilling simulation device and detection instrument cannot be applied to the NGH drilling simulation research and must be designed and manufactured again to ensure high pressure resistance and high test precision.